1. Field of the Invention
The present invention relates to a scroll compressor.
2. Related Art
To reduce the axial gas force (pull-off force) that separates a fixed scroll and an orbiting scroll from each other along a direction of a main shaft which is generated by the compression action of both scrolls, pressure intermediate between discharge pressure and intake pressure is introduced into the backside of the orbiting scroll to produce an attractive force to cancel the pull-off force. Since the intermediate pressure is proportional to the intake pressure, the following problem arises. For example, a shift from a high rotational speed to a low rotational speed causes excess back pressure and hence a large thrust between the orbiting scroll and the fixed scroll. Consequently, sliding friction at top and bottom of each wrap increases to reduce the mechanical efficiency.
In order to solve the problem, Japanese Patent published Application (JP-B) No. 2-60873 (document 1) discloses a scroll compressor in which a back-pressure chamber and an intake space communicate with each other through a valve. Such a structure is provided to let the excess pressure escape.
The pull-off force is determined by a number of factors. One is a pressure distribution of fluid in the compression chambers defined by the orbiting scroll and the fixed scroll while the other is a discharge pressure i.e., a pressure of fluid in a discharge chamber. Since the axial project area of the discharge chamber is smaller than that of all the regions on the side of compression chambers (i.e., the area of a compression chamber which is about to communicate with a discharge port is smaller than the sum of areas of the other compression chambers), except in the case that the number of turns for the scroll wraps is extremely small, the advantage of the discharge pressure on the pull-off force can be omitted to provide a first order approximation. Further, since the compression ratio of the scroll compressor is predetermined in design, the pressure distribution of fluid in the compression chambers (intensity of pressure in individual compression chambers) will substantially depend on suction pressure alone unless an extremely large internal leakage occurs. It is apparent from the above that the pull-off force is generally determined by the suction pressure alone.
On the other hand, the attractive force is exerted for attracting both end plates against the pull-off force. The magnitude of the attractive force is preferably kept at the same level as that of the pull-off force at all times from the standpoint of load-deformation of the scroll members. Although an energizing force exerted between the scroll member and an associated support member is also made small, if relative motion is given therebetween, the danger of friction loss and wear can be reduced. From this point, it is also preferable to keep the attractive force at the same level as that of the pull-off force at all times.
However, since a force from fluid and a centrifugal force are practically imparted to the scroll members in a direction perpendicular to the axis, the attractive force must also resist the inclination moment produced by such forces. For this reason, the attractive force is ideally controlled to be able to attract the end plates of the scroll members with minimum magnitude, but such control can not be realized except in special cases because of an increase in cost.
Therefore, a practical means for applying attractive force has a relatively simple mechanism such that it can realize a force which comprises the pull-off force and a force that can resist the inclination moment throughout the operating range required. As discussed above, since the pull-off force is substantially determined by the suction pressure alone, it is reasonable to provide the attractive force applying means with a mechanism that depends on the suction pressure.
The above document 1 teaches a concrete technique for generating an attractive force by providing a backside excess-suction-pressure region having a pressure dependent on the suction pressure plus a constant value (excess suction pressure value). The scroll compressor is a compressor having a constant capacity ratio. Therefore, as the suction pressure increases, the pressure in compression chambers becomes high in proportion thereto and consequently, the pull-off force increases. Stated more specifically, when the suction pressure increases several times, the pull-off force also increases several times, i.e., by the same factor. In other words, the pull-off force becomes large under the condition that the suction pressure is high. The largest value of the excess suction pressure is thus required in such a condition, and the value is used as the excess suction pressure value in the compressor.
A rated condition in which high performance and reliability are required due to frequent operation is set at about a center of the operating range, and, therefore, the suction pressure also becomes about a center of the range of suction pressure required by operation. For this reason, the suction pressure under the rated condition is extremely different in intensity from the suction pressure with the excess suction pressure value determined for the compressor. In such a case, an excess attractive force causes an increased energizing force between the fixed scroll member and the orbiting scroll member under the rated conditions, so that the danger of sliding friction loss and wear increases to reduce the performance and the reliability.